Plank Speaker

There's something special about an acoustic instrument... you just can't walk past it without banging out a few notes. This project turns a synthesizer or MIDI controller into an electroacoustic instrument, ready to play at the flick of a switch. It's a thin platform on which the keyboard is attached, providing amp and stereo speakers.

Cabinet

The exact dimensions depend on the size of your keyboard, whether you need space for an iPad or synth module, etc. You need to provide an internal volume of 5 liter (2.5 liter per speaker). The prototype I built has external dimensions: 40" x 10" x 1.5" thick, and is built from 1/4" (actually 5mm) birch plywood and 1" x 1" battens.

The bass response of each speaker is augmented using a 1" x 2 cm rectangular vent. The vent is about 16 cm long and is constructed by leaving a gap on each side and adding short internal battens. The tuning frequency is about 55 Hz and the measured bass extends down to 45 Hz.

Interior view showing the separate Left and Right channel speaker enclosures and side vents:

The cabinet is assembled with glue and wood screws. The battens provide a strong base for the screws, though glue and clamps alone would likely have been sufficient. I stained it with a light wash of diluted green food coloring, applied 3 coats of water based polyurethane, and painted the edges black.

Drivers

A pair of Dayton Audio ND91 3.5" full-range Neodymium drivers are mounted face-down into the speaker cabinet, since they are too large to mount internally. Apart from the distinctive appearance, this has the benefit of protecting the fragile aluminum diaphragms. What you hear is the rear radiation from the open driver basket. These drivers have an exceptionally long linear excursion (5mm xmax, with 25mm peak-to-peak travel!) and 30w power handling per channel.

A quick measurement shows bass down to about 45 Hz. The high frequencies are compromised by the inverted drivers, but work perfectly for Rhodes, Wurly, or B3 patches.

Amp

I used a Lepai 2020A stereo Class T amp, chosen because it is cheap and easily mounted. Don't skimp on the power supply since this is the main determinant of the amp's output capability.

Synth/Controller

My original goal was to provide the experience of playing a Rhodes electric piano or Hammond organ. I had hoped to use a hardware synth instead of a MIDI controller, but could not find a cheap one providing a 4 octave range, let alone this range with full-size keys. (If this is not your requirement, then obvious alternatives are a MicroKorg, the Yamaha Reface series, or perhaps a used Alesis Micron.)

I found a used CME M-Keys on eBay, providing 48 full-size keys. (I had to fix some unresponsive keys as explained in this video.) Paired with an old (SIM-less) iPhone running GarageBand, I got a great e-piano/organ synth for under $100.

Assembly

I mounted a power strip along the back and fashioned some aluminum strips lined with weather stripping to hold the keyboard in place. The iPhone (not shown) is mounted with velcro and attached through an adapter to provide USB out, audio out, and charging simultaneously. I also added an amp-style carrying handle. Overall weight including keyboard is about 15 lb.

Conclusion

For its intended purpose -- a portable electroacoustic e-piano -- this project succeeds. The onboard 30w/channel speakers provide plenty of volume for practicing by yourself or with modest accompanying instruments. It won't provide the power of a full standalone keyboard amp let alone a PA, but neither will an upright piano. Sound quality with Rhodes and B3 patches was full and satisfying, and benefits greatly from the stereo arrangement.